1. Field of the Invention
The present invention relates to a radar apparatus applicable to, for example, a car collision alarm device. In particular, the present invention relates to a low-cost simple radar apparatus that receives a reflected signal, converts the signal into a binary signal, and carries out a cumulative statistical operation on the binary signal, to realize high sensitivity and distance measuring capacity. More precisely, the present invention firstly relates to a radar apparatus for detecting a weak reflected signal, secondary to a radar apparatus for detecting and removing interference signals caused by a radar apparatus of an opposite automobile, thirdly to a radar apparatus for accurately measuring the relative speed of an opposite automobile even with a wide transmission pulse, and fourthly to a durable, reliable radar apparatus installed on a vehicle, to speedily detect a target.
2. Description of the Related Art
A radar apparatus is applicable to measuring a distance between vehicles and providing a car collision alarm. The radar apparatus emits a radio or laser pulse signal toward a car running ahead, receives a reflected pulse signal from the car ahead, and calculates a distance between the cars according to an interval between the transmission and reception of the pulse signal. The radar apparatus, therefore, has a transmitter for transmitting the radio or laser pulse signal toward the car ahead and a receiver for receiving the reflected pulse signal and converting it into an electric signal. The radar apparatus still has a controller for controlling the timing of emitting the pulse signal, and a time measurement unit connected to the receiver, for measuring the interval between the transmission and reception of the pulse signal.
The transmitter transmits a pulse signal in synchronization with a trigger pulse that is repeatedly provided at intervals of Tr under the control of the controller. When the amplitude of a reflected pulse signal from an external target exceeds a threshold Vth, the receiver provides a detection signal. This kind of conventional radar apparatus has some problems.
The reflected pulse signal usually includes internal and external noise. Accordingly, the threshold Vth must be relatively high to avoid detection errors due to the noise. The noise is generally random noise having a Gaussian distribution. When the noise has an instantaneous amplitude of n, the probability distribution P(n) thereof is a probability density function demonstrating a Gaussian distribution with an average of zero and a variance of .sigma..sup.2 where .sigma. is a standard deviation. The probability density function P(n) is expressed as follows: ##EQU1##
In the expression (1), the a .sigma..sup.2 is noise equivalent power and the .sigma. is the effective value thereof. When this noise is contained in a signal having an amplitude of s, the probability density function P(n-s) thereof is expressed as follows: ##EQU2## Accordingly, the probability of (n-s).ltoreq.x with t=(n-s)/.sigma. is expressed as follows: ##EQU3## For example, .phi. (1)=0.84134, .phi. (2)=0.97725, and .phi. (3)=0.99865. To correctly detect a reflected signal in the probability of 99.865% from a target at a required distance, the output power of a pulse signal to be transmitted must be determined with a threshold of 3.sigma., and amplitude higher than the threshold by 3.sigma., and a peak value six times larger than the effective value .sigma. of the noise. This corresponds to an S/N ratio of 15.6 dB. According to a radar equation, the level of a reception signal attenuates in proportion to the fourth power of a distance. Accordingly, a long distance is measurable only with an expensive high-power oscillator. Instead of increasing output power, reception strength may be increased. This requires, however, a wide antenna that increases the shape and weight of a radar head, which will be hardly installed on a vehicle. For the safety of the human body, the output power must be low according to safety criteria. This will hardly realize a required detection level.
Japanese Laid-Open Patent Nos. 1-46034 and 2-2106 disclose a method of improving reception sensitivity to a weak signal. This method is applicable to receiving a cyclic signal such as a loran-C signal. The method converts the signal into a binary signal involving positive and negative values. The binary signal is sampled and accumulated in a RAM for a given period under the control of a microcomputer. According to the contents of the memory, the existence, S/N ratio, and occurrence point of the signal are detected. The accumulation of data greatly improves the detectable S/N ratio of a weak signal. If a long detection time is allowed, this method is effective to a signal such as the loran-C signal having a relatively long period. When this method is applied to receiving a radar signal, some problems arise. Since the reception strength of a radar signal is proportional to the fourth power of a distance, sensitivity must be improved 16 times to double a detection distance. To improve the sensitivity by calculation, the improvement is proportional to the one second power of the number of accumulation operations. Accordingly, the sensitivity will be improved 16 times if the number of accumulation operations is increased by 16.sup.2 =256 times. A period of repetitive transmission of a radar pulse must be as short as possible. Since the conventional method employs the microcomputer to control the RAM to accumulate radar pulses, a time necessary for sampling and accumulating the pulses is determined by the clock and instruction cycle of the microcomputer. This results in limiting the period of repetitive transmission of the radar pulse. Accordingly, it is difficult to greatly increase the number of accumulation operations to improve the sensitivity. When the radar apparatus is applied to a car collision alarm device, it raises other problems. If a like radar apparatus is mounted on an opposite car running toward this side, pulse signals of the radar apparatuses of that and this cars will interfere with each other to hinder correct distance measurement. Not only the pulse signal of the opposite car but also engine sparks, ON/OFF operations of a headlight, an air conditioner, and a wiper, fluctuations in a power source voltage, sunlight, and tunnels cause noise. Such internal and external noise will resonate to produce a noise signal that may exceed the threshold. Then, the noise signal will be erroneously detected as a reflected signal from a car that is actually nonexistent.
Japanese Laid-Open Patent No. 3-171380 of this applicant explains that it is necessary to measure not only a distance between cars but also a relative speed between the cars when providing a collision alarm. When measuring the relative speed, the conventional radar apparatus has some problems. The conventional pulse radar is capable of measuring only a distance to a target. Accordingly, to measure a relative speed, the conventional radar must increase accuracy to measure a rate of temporal changes. To increase the accuracy, it is necessary to shorten the width of a pulse transmitted from the radar as well as increasing sampling points. To achieve a measuring accuracy of one meter in a measuring range of 130 meters, 130 sampling points must be prepared. This elongates a time for carrying out accumulation operations. In addition, the width of a pulse to be transmitted must be shortened to several nanoseconds. This complicates a transmitter, increases cost, and deteriorates the performance of the radar.
To secure the durability and reliability of light emitting elements of the transmitter, it is necessary to decrease the duty ratio of pulses. This limits a repetition period of pulses. Namely, if the repetition period is shortened to speedily measure a distance, the durability and reliability of the light emitting elements decrease. On the other hand, if the repetition period is extended, it will take a long time to measure a distance.